split screens in 2.8
It is the same as in 2.79 only the corners now look different - if you click on a corner of one area and then start dragging it in the direction of itself then it splits, if you start dragging it in the direction of another area it joins them. I believe the design for these functions might still be worked on and I would not be surprised if it was made a bit more intuitive in the end.

You can also rightclick on the edge between two areas and choose Join.